This invention relates to write current drivers, and particularly to improvements in H-switches used to supply write current to magnetic recording heads.
H-switch drivers are used in magnetic disk drives to supply write current to the magnetic recording head to write data to recording disks. The H-switch is characterized by two or four semiconductor switches or the like, each forming a leg of the H, with the write head coil being connected to nodes across the center span of the H. A controlled electrode (e.g., collector or emitter) of each semiconductor switch is connected to one of the nodes. A programmed write current is applied to the H-switch and directed in either a first or second opposite direction between the nodes and through the head's coil by selectively operating the semiconductor switches to conduction. During the quiescent state when write current is flowing in a steady state condition in a given direction through the head, the voltages are nearly equal at both nodes, usually about a diode drop below the DC supply voltage to the circuit.
The DC supply voltage is applied to a controlled electrode (e.g., collector or emitter) of the switching semiconductors at the nodes of the H-switch, creating a voltage difference between that controlled electrode and the control electrode (e.g., base) of that semiconductor. Under some circumstances, that voltage can be large enough to break down the transistor, causing a breakdown current (I.sub.BV) to flow out of the base of the transistor. The breakdown current adds to the write current supply, causing inaccuracy of the programmed write current and adversely affecting the performance of the H-switch.
The present invention is directed to a compensation circuit that operates a simulation transistor under conditions that simulate operation of the H-switch transistors susceptible of breakdown, and that senses breakdown of the simulation transistor to generate a compensation current that is combined with the write current to the H-switch.